Video apparently shows rebels executing loyalists of President Bashar Assad in the key northern city of Aleppo, where fighting continues to rage.

BEIRUT — Syrian President Bashar Assad on Wednesday exhorted his military to maintain "continued preparedness," as human rights groups condemned an apparent rebel execution of Assad loyalists in the embattled northern city of Aleppo.

Assad lauded his troops' role in "confronting the criminal terrorist gangs," a reference to the rebels fighting across a wide swath of the country, in a statement marking the anniversary of the founding of the army, state media reported.

Meanwhile, video surfaced online appearing to show the execution of members of a loyalist clan in an Aleppo neighborhood.

In the video, apparent rebels lead a group of prisoners identified as pro-government paramilitary fighters known as shabiha into a courtyard. Several prisoners look bloodied and dazed. One is wearing only boxer shorts. The fighters shout "God is great!" as the prisoners are shoved against a wall. The fighters step back and open fire with their AK-47 rifles for more than 30 seconds, apparently squeezing off hundreds of rounds in a frenzy of shooting and shouting.

Afterward, the camera zooms in on what appears to be four bloodied bodies. Some reports identified the execution site as a school in Aleppo.

The opposition has said that those targeted were members of the Berri clan, a family with close ties to the government. Among those reportedly killed was the clan leader, Zeino Berri. Rebels accused him and others of committing atrocities against civilians. But the executions drew condemnation of the rebel forces. Several opposition groups also publicly denounced the killings.

"Such reprisals are never justified," Nadim Houry of Human Rights Watch said on Twitter.

Some reports indicated that the targeted clan has thousands of followers who could seek revenge, adding to the hostilities in Aleppo.

Rights monitors have documented extrajudicial killings and other abuses by both sides during the Syrian uprising.

Syria's military has suffered heavy losses and numerous defections that have strained its capabilities. A United Nations-commissioned report in June concluded that "regular forces are exhibiting a certain fatigue."

Still, the military retains a substantial edge in training and weaponry compared with the various rebel militias, which are mostly lightly armed, contain many untrained civilians and have no central command.

Government troops and rebels are fighting for control of Aleppo, Syria's most populous city, in what could be a decisive battle in the almost 17-month uprising against Assad's rule.

The U.N. confirmed Wednesday that the military had deployed jets firing rockets and machine guns at rebel positions in Aleppo. The opposition says the government has also used helicopter gunships and artillery to pound rebel-held districts. Rebel commanders say they hope that using guerrilla-style tactics in a dense urban landscape will help neutralize the army's advantage in firepower.

The fight for Aleppo has been brutal and bloody, featuring street fighting for control of neighborhoods and strategic installations such as police stations. Both sides appear to be preparing for a battle that could last weeks.

Tens of thousands of Aleppo residents have fled the city, but aid groups say many people remain trapped. Thousands are said to be living in schools, mosques and parks and at other sites. Aid groups worry that a humanitarian disaster will arise if the combat and shelling spread to other districts of the sprawling city, long Syria's commercial hub.

In Washington, U.S. officials disparaged Assad for keeping his whereabouts unknown since a bombing killed four of his top aides July 18.

"His recent remarks show him for the coward that he is," said Jay Carney, White House press secretary. "He hides out of sight, encouraging the heavily armed Syria military to continue slaughtering civilians in his name."

U.S. officials confirmed that they had increased their aid allocation to the rebels to $25 million, from $15 million. They said the money was for nonlethal aid, notably encrypted radios to help the insurgents remain in touch in the field. U.S. officials say they have set aside $64 million for humanitarian relief in Syria.